Revert "Reverting custom pseudo element".

Source/core/css/CSSSelector.cpp

 /*
  * Copyright (C) 1999-2003 Lars Knoll (knoll@kde.org)
  *               1999 Waldo Bastian (bastian@kde.org)
  *               2001 Andreas Schlapbach (schlpbch@iam.unibe.ch)
  *               2001-2003 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2002, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 465 matching lines @@
     if (name.isNull())
         return PseudoUnknown;
     HashMap<StringImpl*, CSSSelector::PseudoType>* nameToPseudoType = nameToPseudoTypeMap();
     HashMap<StringImpl*, CSSSelector::PseudoType>::iterator slot = nameToPseudoType->find(name.impl());
 
     if (slot != nameToPseudoType->end())
         return slot->value;
 
     if (name.startsWith("-webkit-"))
         return PseudoWebKitCustomElement;
-    if (name.startsWith("cue"))
+    if (name.startsWith("x-") || name.startsWith("cue"))
         return PseudoUserAgentCustomElement;
 
     return PseudoUnknown;
 }
 
 void CSSSelector::extractPseudoType() const
 {
     if (m_match != PseudoClass && m_match != PseudoElement && m_match != PagePseudoClass)
         return;
 
@@ skipping 448 matching lines @@
             return false;
         return (count - m_b) % m_a == 0;
     } else {
         if (count > m_b)
             return false;
         return (m_b - count) % (-m_a) == 0;
     }
 }
 
 } // namespace WebCore